There are billions of planets in our Galaxy and new research suggests that some of these planets have moved from the star systems in which they were formed to find a new home around different stars. 

Planets have been discovered wandering between stars, having been evicted from their home system. This can easily occur in the turmoil of a young planetary system, as the complicated dynamics become unstable. If two planets get too close to each other, then the bully of the pair can shove the other out of the system completely. This may have even happened in our own Solar System billions of years ago, as it is thought that there was once a fifth giant planet.

The new research used computer simulations to create a model star cluster containing free-floating planets. The simulations revealed that if the number of nomad planets was the same as the number of stars, then between three and six per cent of stars would bag themselves an extra planet, and that this is more likely to happen for massive stars.

The astronomers used a young star cluster in their models, as these are more densely packed with stars and planets and thus more prone to gravitational interactions. When a cluster ages, these interactions cause the stars to spread out from each other, making planet capture less likely.

A captured planet would orbit its adoptive star at a great distance, possibly up to thousands of times further than the Earth is from the Sun. It would also likely have a highly tilted orbit compared to the native planets, and it is possible that it would orbit around the star backwards.

However, these unusual orbital characteristics are not solely exhibited by captured planets, as the dynamical games early in a planetary system’s life can also cause native planets to be on unusual orbits. This makes it difficult to distinguish between the two types of planets, and the closest thing found so far is a double planet, discovered in 2006, that has no host star.

"The rogue double-planet system is the closest thing we have to a 'smoking gun' right now," said Hagai Perets of the Harvard-Smithsonian Center for Astrophysics. "To get more proof, we'll have to build up statistics by studying a lot of planetary systems."

Finding a planet in a far-flung orbit around a low-mass star would be a good sign of capture, researchers said, because the star's disc wouldn't have had enough material to form the planet so far out.

The study will be published in The Astrophysical Journal.